1) Field of the Invention
This invention generally relates to the field of hydrocarbon conversion and particularly to the dehydrogenation of paraffinic hydrocarbons to olefinic hydrocarbons, and/or lower alkylaromatic hydrocarbons to vinyl aromatic hydrocarbons. In several preferred embodiments, the invention relates to the dehydrogenation of lower alkanes, for example ethane, isopropane, propane and butanes to their corresponding olefins, for example ethylene, propylene and butylenes; and/or to the dehydrogenation of lower alkylaromatic hydrocarbon compounds, for example ethylbenzene, propylbenzene and methylethylbenzene to their corresponding vinyl aromatic (that is “alkenylaromatic”) hydrocarbon compounds, for example styrene, cumene and alpha-methyl styrene, respectively. The invention further includes an integrated process for making olefinic and vinyl aromatic hydrocarbons including alkylation and dehydrogenation steps.
2) Description of Related Art
U.S. Pat. No. 6,031,143 and its corresponding EP 0 905 112 describe an integrated process for producing styrene by feeding benzene and recycled ethylene to an alkylation reactor to produce ethylbenzene, mixing the alkylation effluent with ethane and feeding the mixture to a dehydrogenation reactor containing a catalyst capable of contemporaneously dehydrogenating ethane and ethylbenzene. The resulting product is separated to produce a stream of styrene and ethylene, with ethylene being recycled to the alkylation reactor. The dehydrogenation reactor is preferably a fluidized bed reactor connected to a fluidized bed regenerator from which the catalyst is circulated between the regenerator and the dehydrogenation reactor in countercurrent flow. That is, catalyst is introduced to the dehydrogenation reactor from the top and slowly descends to the bottom in countercurrent with the gas phase reactants which are rising through the reactor. During this descent, the catalyst is deactivated. The deactivated catalyst is removed from the bottom of the dehydrogenation reactor and transported to the top of the regenerator where it descends to the bottom in countercurrent flow with hot air which is rising. During this descent, the carbonaceous residue present on the catalyst is burnt and the regenerated catalyst is collected at the bottom of the regenerator where it is subsequently circulated back to the top of the dehydrogenation reactor.
WO 02/096844 describes an improvement to this process where the dehydrogenation catalyst is transported from the regenerator to the dehydrogenation reactor by way of a lower alkyl hydrocarbon carrier, for example ethane. During transport, a portion of the carrier is dehydrogenated, (for example ethane converted to ethylene), and the catalyst is cooled.
EP 1 255 719 (and corresponding co-pending US patent publication no. US 2003/0028059, both filed by the assignee of the present application) describes a similar integrated process of preparing styrene using benzene and ethane as raw materials. However, the process includes additional separation and recycling steps that are designed to improve efficiency. For example, the dehydrogenated effluent exiting the dehydrogenation reactor is separated into its aromatic and non-aromatic constituents. The non-aromatic constituents, namely ethane, ethylene and hydrogen are recycled to an alkylation reactor were they are combined with benzene. The aromatic constituents are further separated, for example styrene is recovered and ethylbenzene is recycled to the dehydrogenation reactor. The alkylation effluent is separated into its constituents with hydrogen being removed, and ethane and ethylbenzene being directed to the dehydrogenation reactor. The dehydrogenation reactor may have a variety of conventional designs including fixed, fluidized, and transport bed.
The described dehydrogenation processes are effective at integrating the production of styrene and ethylene using ethane and benzene as the starting materials. Thus, these processes effectively de-coupled the production of styrene from the presence or proximity of a light hydrocarbon steam cracker as a source for ethylene. However, the dehydrogenation processes described employ relatively long contact times between the hydrocarbons and catalyst while at reaction temperature, resulting in thermal cracking, undesired side reactions and the formation of tars and other heavy products.
WO 02/096844 introduces the concept of a split “riser-type” dehydrogenation reactor operating in concurrent or “equicurrent” mode wherein catalyst is carried upwards pneumatically through the dehydrogenation reactor by the gas phase reactants. The space velocity (GHSV) for such a reactor is greater than 500h-1. The catalyst is introduced into the reactor with an alkyl hydrocarbon such as ethane whereas the alkylaromatic compound, for example ethylbenzene, is introduced at a suitable height along the riser after much of the alkyl hydrocarbon has be dehydrogenated and the temperature of the catalyst has been reduced. While no specific examples or operating conditions are provided, the use of such a riser reactor presumably leads to reduced contact times between reactants and catalyst while in the reactor.
Dehydrogenation temperatures and residence times are typically optimized to balance the reaction kinetics of both catalytic and gas-phase (thermal) reactions. The catalytic reaction produces a high selectivity to the desired products while the gas phase reaction produces many undesired products and impurities. That is, while the catalytic reaction kinetics to the desired products increases exponentially with temperature so does the gas phase reaction kinetics; therefore, the proper residence time and reaction temperature profile must be selected to drive both the catalytic reaction to the desired conversion while not allowing the non-selective gas phase reactions to overwhelm the total product selectivity. It would be useful to provide an apparatus and process which minimizes the time period in which reactants and catalyst are in contact with one another while at reaction temperature. This is particularly the case when utilizing highly reactive catalyst which can quickly deactivate.
While not directed toward a “dehydrogenation process” as described in the aforementioned references, WO 03/050065 describes an integrated process for making styrene where benzene and “recycled” ethylene are combined in an alkylation unit with the resulting product stream of ethylbenzene being combined with ethane. Unlike the previously described references, this process utilizes an oxidative dehydrogenation (oxodehydrogenation) reaction. That is, the product stream from the alkylation unit is combined with ethane and oxygen and then contemporaneously oxidatively dehydrogenated to provided ethylene and styrene. The resulting ethylene is recycled to the alkylation unit. The oxodehydrogenation reactor is described as a fluid-bed reactor operating at a temperature range of from 300 to 550° C., a pressure range from 1 to 30 bar, a gas hourly space velocity of 2000 to 6000h-1, with a residence time of the catalyst in the fluid-bed zone of from 1 to 60 seconds.